A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as droplets of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g. EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
Another known method of producing EUV radiation is known as dual laser pulsing (DLP). In the DLP method droplets are pre-heated by a Nd:YAG laser to cause the droplet (e.g., a tin droplet) to decompose into vapour and small particles that are then heated to a very high temperature by a CO2 laser.
In known methods such as LPP and DLP methods a stream of droplets must be generated. The droplets may be generated as either a continuous stream or in pulses.
For example, in one known method that is used in particular for LPP methods a heated container is filled with molten tin that passes from the container to a capillary via a filter and a piezoelectric actuator. A continuous jet issues from the end of the capillary that is modulated in velocity by the piezoelectric actuator. During flight this jet decomposes into small droplets and due to the modulated velocity these smaller droplets merge into larger droplets spaced at larger distances.
In another known method droplets (e.g., liquid tin) are extracted from the end of a nozzle held at high-voltage using an extraction electrode that is switched pulse-wise between the high voltage of the nozzle and ground (or at least to a voltage between the high voltage and ground). The droplets are then accelerated by means of a grounded acceleration electrode and further electrodes are used for adjusting the flight trajectory of the droplets. The pulse length for the extraction electrode is chosen such that the droplet is released completely, has been fully charged and has passed the position of the extraction electrode. From the moment that the extraction electrode is switched back to a high voltage the droplet is accelerated further in the field between the extraction electrode and the acceleration electrode.
A difficulty with such prior methods, however, is that during start up of the droplet generator the speed and direction of the droplets may vary until a steady state operation is achieved. This means that the droplets may land in other parts of the EUV source such as electrodes, sensors and the like. Such contamination—especially for example where the droplets are of a material such as tin—can over time have significant deleterious effects on the operation of the droplet generator.